While a range of devices suitable for detecting radiation fields are known, few if any, are suited to use in a dosimeter that satisfies the demands of contemporary radiation therapy techniques. In order to accommodate contemporary therapy techniques, there is a need for a dosimeter that can accurately verify the radiation treatment therapy, accommodate time dependent therapy techniques such as intensity modulated radiation therapy, and also accommodate precision therapy techniques such as stereotactic radiosurgery. The dosimeter should be able to provide high spatial resolution, while retaining the ability to integrate the total dose over the whole treatment period. The dosimeter should also provide a frequently updated reading of the current radiation dose.
A further requirement for brachytherapy applications is that the dosimeter must be of very small size. A still further requirement is for the dosimeter to be relatively robust, an advantage for any application, but again particularly so if the application requires insertion into patient cavities, for example the urethra.
Fibre optic dosimeters have a number of characteristics that provide advantages over the alternatives for use with radiation therapy techniques. The scintillator of a fibre optic dosimeter, consisting of a small water-equivalent plastic material, avoids disadvantages associated with energy dependence or perturbation of the radiation beam, which occurs with conventional alternative dosimeters.
U.S. Pat. No. 5,006,714 describes a scintillator dosimetric probe. A scintillator is positioned in an ionising radiation beam, which creates light output. The light is conducted from the scintillator through a light pipe to a photomultiplier tube, which converts the light into an electric current. The electric current produced by the photomultiplier tube is proportional to the radiation dose-rate incident upon the scintillator. Through a measurement of the electric current, the radiation dose rate may then be displayed or recorded.
An identified problem with fibre optic dosimeters is the generation of Cerenkov (or Cherenkov) radiation in, and transmission of the Cerenkov radiation along, the light pipe. The intensity of the Cerenkov radiation is dependent on factors other than the radiation dose at the scintillator and therefore the Cerenkov radiation represents noise in the measurement signal.
One proposed technique to address the problem of Cerenkov radiation is to use signal processing. United States patent application publication number 2004/0238749 proposes a method for measuring a dose of radiation that involves filtering using two band pass filters.
A problem with the use of signal processing to discriminate between Cerenkov radiation and the signal from a scintillator, is obtaining sufficient discrimination in order to provide the required accuracy of measurement. This problem is particularly apparent when the most effective scintillators emit light at a wavelength that occupies a similar area in the electromagnetic spectrum to Cerenkov radiation.
Another problem with the use of fibre optic dosimeters is to measure radiation intensities that are calibrated, that is the reading is referable to the reading of a standard dosimeter in the same radiation field. The reading is not just dependent on the scintillation signal, but also on the losses between the scintillator and detector. IN practical use there losses can vary, for instance connectors for optical fibres vary in their efficiency. If the dosimeter is disconnected from the detector and reconnected, the reading may change. In addition, the efficiency of common light detectors may change in time as well as vary from one detector to another.
It is therefore an object of the present invention to provide a dosimeter that satisfies one or more of the aforementioned needs and/or overcomes or alleviates at least some of the problems of existing dosimeters, or at least one that provides the public with a useful alternative.
It is a further or alternate object of the present invention to provide a method of manufacture of a fibre optic dosimeter that results in an improved dosimeter or at least one that provides a useful alternative.
In this specification, any reference to the prior art does not constitute an admission that such prior art is widely known or forms part of the common general knowledge in Australia or in any other jurisdiction.